1. Technical Field
This invention relates to a torsion bar with variable spring constant.
2. Background Art
A torsion bar with variable spring constant is known in the art. One example is disclosed in Japanese Utility Model Application, Publication No. 62-16505, which is schematically illustrated in FIG. 13 of the accompanying drawings. As shown in FIG. 13, the torsion bar `a` has a first bar member `b` and a second hollow bar member `c` coaxially extending over the first bar member until a joint portion `d`. These bar members `a` and `b` are fixedly connected to each other at the joint portion `d`. The other ends of the bar members `b` and `c` are connected with each other via a slidable coupler `e`. The coupler `e` can slide along the axis of the bar member `b` in the bar member `c`. In this arrangement, pulling a cable `f` along the axis of the bar member `b` changes a "torsion length" L which determines how much the bar members can twist or rotate about their own axes. More specifically, as the slidable element `e` MOVES toward the fixed joint `d`, that part of the hollow element `c` which is right to the element `e` in FIG. 13 does not work as a torsion bar element so that the total stiffness of the torsion bar `a` is reduced. When the slidable element `e` reaches the fixed joint `d`, only the bar member `b` works as a torsion element. In this manner, the spring constant of the torsion bar `a` is varied by manipulation of the cable `f`.
This prior art arrangement, however, cannot achieve great change in the spring constant because the spring constant or bar rigidity is changed by means of the coupler `e`. The spring constant can only be changed from a value representing the spring constant of the bar member `b` at minimum (when the coupler `e` takes the leftmost position in FIG. 13) to another value representing the total or combined spring constant of both bar members at maximum (when the coupler `e` takes the rightmost position). Further, the configuration has many mechanical components and may therefore be more affected by aging than an electrically controlled arrangement. In addition, if it is used as a stabilizer bar, as shown in FIG. 13, a great amount of load will be applied onto the joint portion `e`. Also, it may be very difficult to mount the torsion bar on an automobile body, considering the complexity of the cable arrangement, because the coupler `e` is designed to be pulled via the cable `f`.
Another torsion bar with variable spring constant is disclosed in Japanese Patent Application, Publication No. 1-278815, which is schematically illustrated in FIG. 14 of the accompanying drawings. One end of the torsion bar `g` with variable spring constant is fixed to a suspension arm `h` and the other end is fixed to a bracket `i`, which is mounted to the body frame. Attached at an intermediate portion `j` is a support mechanism `k` to prevent or allow torsion (twisting motion) of the torsion bar `g` relative to the body frame. In this configuration, fixing the intermediate portion `j` of the bar `g` by means of the support mechanism `k` reduces a substantial torsion length from L2 to L1 and thus varies the spring constant, while releasing the intermediate portion `j` extends the virtual twisting length of the bar `g` to L2 and thus decreases the spring constant.
The above-described arrangement was intended to be used as a torsion bar for automobile suspensions. However, the total length of the torsion bar `g` becomes too long to be mounted onto the body frame if a softer suspension is desired. Also, it can only change the spring constant between two values L1 and L2, i.e., higher and lower ones. If more than two variable spring constants should be realized, more than one support mechanisms `k` must be provided on the bar `g`. In this case, however, the number of manufacturing processes increases because each support mechanism `k` must be separately mounted on the body frame. In addition, each support mechanism applies a vertical load to the body frame, increasing the number of input points of the vertical loads applied to the body frame. Thus, time-consuming structural analysis must be conducted to find out how much loads act at the respective input points and whether the strength of the body frame is sufficient.